Electric vehicles and the grid

Increased EV uptake means challenges for the grid, but the good news is that demand-side management could help. Marcus Brazil and Julian de Hoog explain.

The last few years have seen a rapid increase across the globe in the uptake of electric vehicles (EVs). A recent report by the US Department of Energy points out that sales of EVs are increasing faster than those of hybrids, if you compare the same stages of the technology life cycle. One of the leading manufacturers of EVs, Tesla, is now valued at more than $30 billion and has been projected to be a major disrupter of the automotive industry. Many major car makers are planning to introduce electric and hybrid models into the Australian market in the near future.

An important question that is often overlooked in discussions on electric vehicles is: what will be their impact on the electricity grid? An EV with a typical daily commute of 40 km requires roughly 6–8 kWh of energy to recharge; this is equivalent to the daily needs of a small household. In other words, if you buy an electric vehicle, the impact on the local electricity network is about the same as adding a small house to the neighbourhood. Furthermore, in an unregulated environment, most EV owners are likely to plug in when they arrive home, around 6 pm, at exactly the time that residential electricity networks experience peak demand.

The impact of EVs on the electricity grid

The main challenges to the grid arising from the increased uptake of electric vehicles fall into three categories.

Peak load

Perhaps the most well known of the problems, peak load occurs at the time of day when the highest demand for electricity occurs. Our networks are sized to withstand peak demand, but if electric vehicles are added to the network, then the additional demand may be too much for the local lines and transformer to handle. New infrastructure may need to be installed, the cost of which is inevitably borne by the consumer.

Voltage drop

The lines in a local distribution network have an impedance of their own. As current travels from the transformer to your house, this impedance leads to a decrease in voltage. The more current you draw, the more the voltage drops. An electric vehicle can draw a lot of current for long periods of time, and therefore cause a significant, sustained drop in your voltage. Low voltage can mean that some appliances may not run properly, or may run inefficiently (reducing their lifetimes). What’s more, the drop in voltage caused by an EV does not affect only its owner; it can affect other houses in the network too, particularly those furthest from the transformer.

Phase unbalance and power quality

Most distribution networks in Australia are 3-phase, and most houses connect to only one of these phases. If several people in a neighbourhood buy EVs, and by chance these are connected to the same phase, then there can be significant losses in efficiency in the network due to the resulting unbalance. There is also the potential that an EV and its charge point could affect the overall quality of the power in the network, for example by distorting the 50 Hz grid waveform.

When are these problems likely to arise? A study of two networks in Australia suggests that there can already be problems at fairly low vehicle uptake rates—for example at only 10% in a network based in Melbourne. While even a 10% uptake of EVs is some years away, now is a good time to start thinking about how to prepare for these problems.

The solution: shifting of demand

The good news is that many of these problems can be prevented. Electric vehicles are among the most ‘flexible’ loads in the grid: they can be shifted to other times of the day, such as overnight, when there is more capacity in the network.